1. Field of the Invention
The present invention pertains to an adhesive composition suitable for connecting plastic substrates used in plastic liquid crystal displays (PLCDs), etc.
2. Description of the Related Art
Thermosetting anisotropic conductive epoxy-adhesive films have been known and used for practical purposes as a connecting material to be used around the edges of liquid crystal displays (LCDs).
Moreover, plastic liquid crystal displays wherein a plastic substrate has been substituted for the glass substrate of conventional liquid crystal displays have recently been developed in order to make liquid crystal displays lightweight. As a connecting material for such a plastic substrate, conventional thermosetting anisotropic conductive epoxy-adhesive films have also been used.
However, plastic substrates used in conventional plastic liquid crystal displays have inferior heat resistance when compared to glass substrates. On the other hand, the connecting temperature for conventional thermosetting anisotropic conductive epoxy-adhesive films is around 200xc2x0 C. Therefore, there is a problem that when conventional thermosetting anisotropic conductive epoxy-adhesive film is used to connect plastic substrates, the substrates will deform and good quality is not obtained.
Against the problem, a low-temperature-setting anisotropic conductive film of a radical polymerization type of which the setting temperature can be 100 to 140xc2x0 C. has been proposed. However, although deformation of the substrate can be controlled by use of this low-temperature-setting anisotropic conductive adhesive film, sufficient connection reliability is not obtained. This is apparently because the low-temperature-setting anisotropic conductive film of a radical polymerization type has a higher coefficient of cure shrinkage than a thermosetting anisotropic conductive epoxy-adhesive film and therefore, internal stress readily remains and peeling readily occurs after aging, particularly after aging under high humidity.
Moreover, photosetting anisotropic conductive adhesive films of a radical polymerization type that utilize ultraviolet rays are also known as connecting materials with a lower connecting temperature than conventional thermosetting anisotropic conductive epoxy-adhesive, but similarly to the above-mentioned low-temperature-setting anisotrpic conductive adhesive film, sufficient connection reliability cannot be obtained even though thermal deformation of the substrate can be prevented.
The present invention solves the above-mentioned problems with related art, its object is to obtain a connecting material, such as anisotropic conductive adhesive film or the like, by use of which it is possible to obtain sufficient connection reliability, even for connecting plastic substrates that are used in plastic liquid crystal displays.
The inventor completed the present invention upon discovering that when oxetan compounds are mixed in adhesive compositions of a cationic photopolymerization type, reactivity of the adhesive composition can be increased and curing time markedly shortened, and connection reliability can be increased and even plastic substrates with low heat resistance can be well connected.
That is, the present invention presents an adhesive composition, comprising an insulating resin, a photopolymerization initiator, and an oxetan compound.